"The vast majority of that would be left in the ground if one could not hydraulically fracture these wells," said J. Greg Schnacke, executive vice president of the Colorado Oil & Gas Association.

But the sand itself is becoming scarce, putting pressure on already-rising natural-gas prices.

Also called stimulation, fracing involves shooting a mix of sand, water and chemicals into gas wells to crack brick-like sandstone, releasing gas trapped in tiny pores dating back to prehistoric times.

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The sand must be hard, evenly sized and nearly spherical to let the gas get by. Colorado sand, with its uneven grains, would block the pores.

The good sand is smooth to the touch and creamy in color, like something from a Caribbean travel poster. It's mined in the Great Lakes region, and Union Pacific has been rolling it into the Rockies in covered hopper cars for decades.

"In Colorado, we estimate something slightly over 100 trainloads of sand per week are coming into the state. That's 200 tons per trainload," Schnacke said.

Frac sand was smoothed by waves up to 500 million years ago, said Genevieve Young, petroleum geologist with the Colorado Geological Survey.

Colorado, too, is awash with ocean sand, such as the Dakota Hogback sandstone that Interstate 70 cuts through west of Denver, she said. But Colorado's seashore vanished too soon, leaving other minerals mixed in with the silica.

The champion of frac

The Rockies' fracing champ is Halliburton, said Craig Van Kirk, head of the petroleum engineering department at Colorado School of Mines.

Long before Iraq put the company in the headlines, Halliburton crews were famed locally for their ability to squeeze gas out of seemingly dry earth.

As a services company, Halliburton doesn't own wells but gets called in when an exploration and production company thinks fracing is the way to go. Halliburton brings its own sand, plus trailers full of computerized gear to mix the fluid and control the speed at which it's pumped into the earth.

"Right now we're going through, I would say, close to 4 million pounds a week. I've been with Halliburton for 29 years, and we've been pumping this sand ever since I've been here," said Ron Rafferty, a Halliburton account manager.

The job usually takes a day. By sundown, the owner has a well that may produce gas for years.

"The charge for that sand is part of their overall invoice to us as one of the products that they provide as part of the stimulation job," said Alan Harrison, vice president of Kerr-McGee Rocky Mountain Corp., which often uses Halliburton.

The local Halliburton office orders three weeks' worth of frac sand at a time from suppliers in Illinois and Minnesota. At $15 per 100-pound sack, that's $1.8 million worth of sand.

Fracing is more common in the U.S. than elsewhere in the world, and more common in the Rockies than elsewhere in the U.S., Van Kirk said. Americans have been drilling longer than the rest of the world and have already sucked out the easy gas, making fracing worth the effort and expense - $30,000 to as much as $5 million per well.

The gas that's left in Colorado and nearby states is stuck in sediment that's harder and less permeable than elsewhere.

The whole idea is to increase the supply of natural gas.

"If it wasn't for fracing, gas prices would be an awful lot higher," said Wade Peters, manager of business development and engineering for the Denver office of Calfrac Well Services Corp., a Canadian company that fracs in northeast Colorado and the Western Slope.

"Demand for sand is way up, so we're scrambling to get alternate sand sources to keep working," he said.

Finding the right stuff

The best natural sand for Colorado conditions is 425 to 850 micrometers in diameter and comes from the St. Peter sandstone formation in Ottawa, Ill., said Jennifer Miskimins, a School of Mines geologist.

There's 50 to 75 years' worth of Ottawa sand left, but producers got stuck with unsellable sand during the energy bust of the 1980s and now intentionally under-produce, Calfrac's Peters said. There may be a 600,000-ton shortage this year.

"They could produce more if they were going to spend the capital to get the stuff out," he said.

"It's not like you go to your local beach and pick up some sand," Peters said.

Artificial sand made of ceramic beads can be used when natural sand isn't available or when a well is so deep that pressure would crush natural sand. Wells in southern Wyoming can reach 14,000 feet, while Colorado wells are rarely deeper than 7,000 feet, Miskimins said.

Fracing can be scary to those who don't understand it, Miskimins said. "You're cracking the Earth, and that freaks a lot of people out."

Fracing can barely be picked up by seismological sensors, Calfrac's Peters said. "It's not like the Earth is going to open up and cities are going to get swallowed."

Yet environmentalists are suspicious, because fracing fluid sometimes contains diesel fuel as a lubricant to help the sand get into the fractures, said Gwen Lachelt, executive director of the Oil & Gas Accountability Project in Durango.

Lachelt said she believes diesel has contaminated groundwater, a possibility the Environmental Protection Agency is studying.

"I will have folks call me up from around the country saying, 'Wow, they just fraced a well near my house, my house shook, my water turned black,"' Lachelt said.

"Then they'll call a week later and say, 'My water's clear now. I think it's safe to drink.' What am I to say? There's a lot of concern about it."

The EPA's draft report suggests contamination is a remote possibility. "Although the threat to public health from hydraulic fracturing appears to be low, it may be feasible and prudent for industry to remove any threat whatsoever from injection of fluids," it says.

But Miskimins said gas wells never touch aquifers. "It physically can't happen. There's too many geologic barriers between the wells that we frac and the water table."

Even when diesel is injected into a well, "that's no different than the hydrocarbons you're handling when they come out," she said. As for the fracing sand, once it completes its voyage from the Great Lakes to a hole in the ground in Colorado, its job is done.

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